The invention relates generally to offshore drilling systems. More particularly, the invention relates to a dual-gradient offshore drilling system using low-density liquid lift for drilling risers.
The search for crude oil and natural gas in deep and ultra-deep water has resulted in greater use of floating drilling vessels. These vessels may be moored or dynamically-positioned at the drill site. Deep water drilling typically involves the use of marine risers. A riser is formed by joining sections of casing or pipe. The riser is deployed between the drilling vessel and wellhead equipment located on the sea floor and it is used to guide drill pipe and tubing to the wellhead and to conduct a drilling fluid and earth-cuttings from a subsea wellbore back to the floating vessel. A drill string is enclosed within the riser pipe. The drill string includes a drilling assembly that carries a drill bit.
A suitable drilling fluid (commonly called xe2x80x9cdrilling mudxe2x80x9d or xe2x80x9cmudxe2x80x9d) is supplied or pumped under pressure from the drilling vessel. This drilling mud discharges at the bottom of the drill bit. Mud lubricates and cools the bit, and lifts drill cuttings out of the wellbore. In conventional offshore drilling, drilling mud is circulated down the drill string and up through an annulus between the drill string and the wellbore below the mudline (sea floor), and from the mudline to the surface through the riser/drill string annulus.
Drilling mud is very important in the drilling process. It serves as: (1) a lubrication and heat transfer agent; (2) a medium to carry away and dislodge pieces of the formation cut by the drill bit; and (3) a fluid seal for crucial well control purposes. To maintain well control, drilling operators attempt to carefully control the mud density at the surface of the well to avoid many potential problems. One potential problem is xe2x80x9clost circulationxe2x80x9d when a column of drilling mud exerts excess hydrostatic pressure, which propagates a fracture in the formation. Formation fluids may enter the wellbore unexpectedly when the hydrostatic pressure falls below the formation pressure. Such an event is called xe2x80x9ctaking a kick.xe2x80x9d A blowout occurs when the formation fluid enters the wellbore in an uncontrolled manner. Both of these problems become even more difficult to overcome in deep water. In a conventional drilling system, the relative density of the drilling mud over that of the seawater, along the length of the riser in deep water, combined with a low overburden pressure, results in excess hydrostatic pressure in the riser/drill string annulus and the wellbore/drill string annulus.
Because of the narrow margins between pore pressure (formation fluid pressure) and fracture pressures (leak-off/lost circulation pressures), equivalent circulating density (ECD) is tightly controlled by balancing hole cleaning requirements and circulation rates. The wellbore is also cased off at frequent intervals to maintain well control.
One solution to these problems known in the art is dual-gradient drilling. Dual-gradient drilling is an area of technology that is primarily used to overcome the narrow pore pressure/fracture gradient margins found in abnormally pressured, ultra-deepwater wells. As an enabling technology, dual-gradient drilling permits drilling in deep and ultra deep water using fewer casing strings than possible using conventional drilling systems. Because there are fewer casing strings used, there is potential for drilling dual-gradient wells faster than conventionally drilled wells. Dual-gradient drilling can also enhance extended-reach drilling by reducing the influence of circulating pressure losses on bottom-hole pressure. Dual-gradient drilling can be used to drill a wellbore with a larger diameter hole at the bottom of the wellbore, resulting in lower pressure drop per unit length than a smaller diameter wellbore.
Forms of dual-gradient drilling technology being developed include pump-lifted and gas-lifted drilling risers. Pump-lift systems use pumps positioned near the sea floor to pump the heavy mud/drilling returns from the mud line to the drilling vessel to reduce the hydrostatic pressure at the mud line, generally to that which would result from a sea water gradient. Illustrative of the pump-lift systems is U.S. Pat. No. 4,813,495 to Leach that discloses a method and apparatus for drilling subsea wells in water depths exceeding 3000 feet (915 meters) (preferably exceeding 4000 feet (1220 meters)) where drilling mud returns are taken at the seafloor and pumped to the surface by a centrifugal pump that is powered by a seawater driven turbine. See also U.S. Pat. No. 4,149,603 to Arnold and published PCT application WO9915758. Limitations with the pump-lift systems include wear and equipment reliability for the subsea pumps and motors. Also, the ability of the pump-lift system to handle dissolved and entrained gas is potentially very poor.
Gas-lift systems use air or nitrogen to xe2x80x9cliftxe2x80x9d the drilling returns, effectively lowering the riser hydrostatic pressure to a seawater pressure gradient. For example, U.S. Pat. No. 4,099,583 to Maus discloses an offshore drilling method and apparatus which are useful in preventing formation fracture caused by excessive hydrostatic pressure of the drilling fluid in a drilling riser. One or more flow lines are used to withdraw drilling fluid from the upper portion of the riser pipe. Gas injected into the flow lines substantially reduces the density of the drilling fluid and helps provide the lift necessary to return the drilling fluid to the surface. The rate of gas injection and drilling fluid withdrawal can be controlled to maintain the hydrostatic pressure of the drilling fluid remaining in the riser and wellbore below the fracture pressure of the formation. See also U.S. Pat. No. 3,815,673 to Bruce, et al., U.S. Pat. No. 4,063,602 to Howell, et al. and U.S. Pat. No. 4,091,881 to Maus. Limitations with the gas-lift system include inefficient or ineffective cuttings transport, dealing with pressurized equipment on the drilling vessel, and detection of fluid influx from the formation to the well bore (kick detection).
Generally, the invention is a method of drilling a well below a body of water using a drill string that starts by injecting into the well, at a depth below the water surface, a liquid having a lower density than a density of a drilling mud. This produces a mixture of drilling mud and low-density liquid in the well. The low-density liquid may be miscible or immiscible with the drilling mud. The mixture of drilling mud and low-density liquid is withdrawn from an upper end of the well. At least a portion of the low-density liquid is separated from the mixture of drilling mud and low-density liquid, with at least a portion of the separated low-density liquid returned to the depth below the water surface and at least a portion of the drilling mud depleted of low-density liquid being returned to an upper end of the drill string.
An embodiment of the invention includes controlling the injection rate of the liquid. First, the rate of the liquid injected can be selected so the cuttings within the riser pipe have an upward velocity in excess of the settling rate of the cuttings in the riser pipe. Secondly, the rate of the liquid injected can be selected so the liquid lift maintains a bottom-hole pressure that is below the fracture pressure of the earth formation and above the pore pressure of the formation.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.